Mechanicals

Check that waste pipes slope enough to flow (as a general guideline, about 1" per 4'). Make sure old plumbing was removed, not just disconnected. Cluttered plumbing, mix-matched materials, and dead-ends may indicate a poor job.

Look for signs of neglected plumbing such as leaky joints and dented/kinked pipes and always check the condition of the water main connection coming into the house.

visible pipes: no damage, no evidence of leaks, no signs of water stains on materials near pipes, drain pipes slope slightly down towards outlet

Check the service panel (a.k.a. breaker box). On both fuse and breaker boxes, the main disconnect should have a written rating of amperage (100 - 200 amps). Older homes may be below the 100 amp minimum and need to be updated to code.

See if the lights dim when running an appliance. This indicates a problem with the service load and/or wiring. Make sure wiring appears to be in good condition and there are an adequate number of wall outlets.

visible wiring: in good condition, no "knob-and-tube" wiring, no exposed splices

service panel: adequate capacity, all cables attached to panel with cable connectors; fuses or breakers are not overheating

If a house has older forced-air heating/cooling systems, they will eventually need to be replaced. This is a significant expense that should be factored into costs.

Steam heat and radiators can actually be pretty efficient when updated with a new boiler, so don't rule them out. Forced air ducts can also be modified to accept a cooling unit or even a fresh air exchange unit (Energy Recovery Ventilator).